Sale Report is now an QWEB(HTML) report not an RML, you can modify in setting->Actions->Reports, search for 'Quotation' select the sale.order one, there you can:

1. change the report type to HTML, print a quotation/sale, and edit it with the website editor.
2. click in Search associated QWEB views, select the report_saleorder_document, and edit it.
3. change it via custom module.
    

for more info go to the officual documentation:https://www.odoo.com/documentation/8.0/reference/reports.html

Hi PY! 

At the final, You have solved the problem? 

For modify PDF. I didn't find exact one for modify PDF reports, but I found a PDF page modify guide: http://www.rasteredge.com/how-to/csharp-imaging/pdf-page-modify/ , you can check whether it's helpful to you.

Best regards.